Cryogenic preservation of biological material is very important in many medical and pharmaceutical fields. There are two aspects to cryogenic preservation of biological samples. The first is the preparation of the fresh sample for freezing and the controlled rate freezing of the sample. The second aspect is the long-term storage of the samples in a cryogenic state. Some cryogenic sample storage systems use an expendable cryogen such as liquid nitrogen to keep the samples cool and to absorb heat leak. The problem is that extensive infrastructure (vacuum piping etc.) and maintenance (monitoring and delivery of liquid nitrogen etc.) are required to keep these systems operating. Mechanical refrigeration systems are available for the low temperature (non-cryogenic) storage of samples. These commercial units do not attain temperatures as low as the liquid nitrogen storage systems (−140° C. for mechanical and −196° C. for liquid nitrogen). These units do not require any expendable cryogen to maintain their refrigeration. Current mechanical refrigerators are very large and inefficient due to significant heat leak through their foam insulated walls. Improved insulation and decreased heat leak into these mechanical refrigeration systems could reduce the required refrigeration capacity by an order of magnitude. Cryogenic sample storage systems are becoming increasingly popular due to increased sample viability and life span at the lower storage temperatures. These systems are more costly to operate and are less convenient due to the required regular supply and handling of liquid nitrogen. Sizes of standard cryogenic storage refrigerators are limited by safety concerns and the necessity of conserving liquid nitrogen by minimizing heat leak. Access is through the top. Units must be thawed periodically to clean-up infiltrated atmospheric contamination. Thawing is labor intensive, incurs the risk of thawing the samples and is hazardous. Size limitations may require the use of many individual units sacrificing any economies of scale.
Accordingly, it is an object of this invention to provide an improved refrigeration system which may be used to refrigerate biological material.